1. Field of the Invention
The invention disclosed in the present specification relates to a substrate on which a thin film semiconductor device typified by a thin film transistor is formed and to a method for fabricating the substrate. The invention also relates to a thin film transistor formed on an insulating substrate such as a glass substrate and a quartz substrate and to a method for fabricating the same.
2. Description of Related Art
Hitherto, there has been known a technology for fabricating a thin film transistor (hereinafter referred to as a TFT) on a glass or quartz substrate. This technology is required to fabricate an active matrix type liquid crystal display.
Presently, the mainstream thereof is a-Si TFT using an amorphous silicon film. However, the a-SiTFT is applied only in constructing an active matrix circuit because its operating speed is slow.
A structure in which a peripheral driving circuit and other circuits, beside the active matrix circuit, are integrated on one glass substrate is being proposed lately. This structure is called a system-on-panel. The structure called the system-on-panel is a configuration required in miniaturizing and lightening an apparatus comprising the liquid crystal display. It is also useful to integrate the circuits having the various functions on one substrate in simplifying fabrication steps and operation checks.
When a TFT is fabricated on the insulating substrate such as the glass substrate or the quartz substrate however, its characteristic has been questionably low and varied. When the characteristic is low, characteristic of a circuit comprising such a TFT is also degraded. When the characteristic varies greatly, characteristic of a circuit comprising such a TFT varies and is degraded.
The grade of the characteristic is related mainly to physical properties of a semiconductor film to be used. The characteristic of the TFT may be enhanced by using a silicon film having high crystallinity.
Meanwhile, the variation and the instability of the characteristic of the TFT are considered to be caused by:
1) instability of process; and
2) electrical instability of a thin film semiconductor obtained.
The variation and the instability of the characteristic of the TFT of an active matrix liquid crystal display are considered to be caused by impurity mixed in steps for fabricating the TFT because:
1) a glass or quartz substrate containing impurity in high concentration as compared to a semiconductor substrate is used as a substrate in the active matrix type liquid crystal display; and
2) the size of a deposition system and a conveyor system increases in correspondence to the substrate having a relatively large area.
For instance, impurity mixed into a semiconductor film composing the TFT in steps for depositing it causes the variation and the instability of the TFT characteristic.
Then, the inventor et. al. have measured film quality and impurity of a gate insulating film which has a large influence on the characteristic of the TFT to study the above-mentioned relationship between the characteristic of the TFT and the variation.
FIG. 11 shows data concerning to impurity existing at the interface between a gate insulating film and a gate electrode of a TFT formed on a Corning 1737 glass substrate. This data is a result of the measurement carried out by means of EDX (energy distributed X-ray micro-analysis). EDX senses elements existing in the order of more than 0.1%. Accordingly, elements detected by the EDX analysis means to exist in the order of more than 0.1% (percentage of number of elements).
This sample is what a silicon oxide film deposited by means of plasma CVD is used as the gate insulating film and aluminum deposited by means of sputtering is used as the gate electrode.
Accordingly, peaks of silicon (Si), oxygen (O) and aluminum (Al) are seen in FIG. 11. However, peaks of trace amounts of barium (Ba) and calcium (Ca) are also seen.
Although the vertical axis of the measured value shown in FIG. 11 is not what reflects the percentage of elements accurately, it shows a relative relationship of the density of their existence.
While counted numbers of barium and calcium are not so large as compared to those of aluminum and silicon in FIG. 11, their density is considered to be high when their electrical influence is considered (they exist at least more than 0.1%.
Barium and calcium are liable to be ionized. Accordingly, such elements existing at the interface between the gate insulating film and the gate electrode with concentration of more than 0.1% may become a significant factor of destabilizing the operation of the TFT.
FIG. 12 shows a result of analysis of the Corning 1737 glass substrate utilized as a substrate implemented by the same measuring method with that shown in FIG. 11.
As it is apparent from FIG. 12, this glass substrate contains relatively high concentration of barium and calcium. It can be considered from this fact that barium and calcium shown in FIG. 11 are what have turned around from the glass substrate used as described above.
This turn-around of the impurity from the glass substrate is considered to have occurred when the substrate is sputtered in depositing the gate electrode, flying around the impurity within the ambient atmosphere.
Further, quartz is required to use as a substrate in implementing heat treatment in a temperature as high as 800xc2x0 C. and 900xc2x0 C. However, although a substrate composing a semiconductor device is required to have high impurity as described above, such a quartz substrate is expensive in general.
A number of ranks exists among quartz substrates and a quartz substrate in the lower rank is inexpensive. However, a quartz substrate in the lower rank contains high concentration of OH group and the OH group affects the operation of the semiconductor device fabricated on the substrate. For instance, it becomes a factor of shifting a threshold value of the TFT on the minus side.
In general, the OH group within the quartz substrate is a factor of destabilizing the operation of the semiconductor device fabricated on the substrate and of varying the characteristic of the device.
According to the measurement carried out by the inventors et. al. of the present invention, concentration of OH within the low grade quartz substrate has been higher than that of a higher grade quartz substrate by more than 15 times.
There has been known a crystal grass (called also as a ceramic glass) as an inexpensive glass substrate having a high heat resistance and a distortion point of 700xc2x0 C. or more. However, because the crystallized glass substrate also has various components, it is feared that impurity may diffuse from the substrate in the process for manufacturing the semiconductor device.
Accordingly, based on the recognition described above, it is an object of the invention disclosed in the present specification to construct a substrate for a semiconductor device which allows a stable semiconductor device having no variation of characteristic to be obtained by preventing impurity from turning around to the semiconductor device from a glass or quartz substrate (or another adequate substrate), and to provide a method for fabricating such a substrate.
It is another object of the invention to provide a semiconductor device whose variation of characteristic and instability have been eliminated and to a method for fabricating such a semiconductor device.
In order to solve the above-mentioned problems, a glass or quartz substrate is used as a substrate of a semiconductor device according to the present invention. As the glass substrate, a Corning 1737 glass substrate, a 7059 glass substrate, an Neoselum N0 glass substrate, an N11 glass substrate and a crystallized glass (ceramic glass) substrate whose distortion point is 700xc2x0 C. or more (typically about 950xc2x0 C. to 1100xc2x0 C.) may be used.
While there are various kinds of crystallized glass, basically aluminosilicate glass and boro-silicate glass mainly composed of quartz (SiO2) and alumina (Al2O3) may be practically used. As a substrate of a semiconductor device, it is preferable to be a non-alkali glass. Mgxe2x80x94Al2O3xe2x80x94SiO2 group, PbOxe2x80x94ZnOxe2x80x94B2O3 group, Al2O3xe2x80x94B2Oxe2x80x94SiO2 group, ZnOxe2x80x94B2O3xe2x80x94SiO2 group and the like are preferable. Specifically, it is required to use a high heat resistant substrate such as the quartz and crystallized glass substrate which can sustain high heating temperature.
(1) In order to solve the above-mentioned problems, according to an arrangement of the invention disclosed in the present specification, a substrate of a semiconductor device is a glass or quartz substrate which surrounding surface is covered by a blocking layer.
(2) According to another arrangement of the invention, a substrate of a semiconductor device comprises a glass or quartz substrate; a blocking layer formed so as to cover the surrounding surface of the substrate; and a silicon film formed so as to cover the blocking layer.
In the above-mentioned arrangements (1) or (2), a film selected from a silicon oxide film, a silicon nitride film and a silicon oxide nitride film is used as the blocking layer.
Further, a film represented as SiXC1-X (0 less than X less than 1) may be used instead of the silicon film. Another semiconductor film may be also used.
(3) According to another arrangement of the invention, there is provided a method for fabricating a substrate of a semiconductor device, comprising steps of depositing a blocking layer on the surrounding surface of a glass or quartz substrate by means of reduced pressure thermal CVD; and depositing an amorphous silicon film so as to cover the blocking layer by means of reduced pressure thermal CVD.
(4) According to another arrangement of the invention, there is provided a method for fabricating a substrate of a semiconductor device, comprising steps of forming a blocking layer on the surrounding surface of a glass or quartz substrate by means of reduced pressure thermal CVD; and depositing an amorphous silicon film so as to cover the surrounding surface of the blocking layer by means of reduced pressure thermal CVD.
In the above-mentioned arrangements (3) and (4), a film selected from a silicon oxide film, a silicon nitride film and a silicon oxide nitride film may be used as the blocking layer.
Further, a film represented as SiXC1-X (0 less than X less than 1) may be used instead of the silicon film. Another semiconductor film may be also used.
(5) According to a still other arrangement of the invention, there is provided a method for fabricating a semiconductor device, comprising steps of depositing an amorphous silicon film at least on a main surface of the substrate; transforming the amorphous silicon film into a thermal oxide film by heating in oxidizing atmosphere; and forming a thin film semiconductor device on the thermal oxide film.
As the substrate, typically a quartz or crystallized glass substrate may be used. The present invention is useful especially in using the crystallized glass or the lower grade quartz substrate.
At least the main surface means the face on which the thin film semiconductor device (e.g. a thin film transistor) is formed. It is possible to arrange such that the amorphous silicon film is deposited also on the back of the substrate and to transform it into a thermal oxide film as shown in an embodiment y.
The oxidizing atmosphere may include, for example:
(i) atmosphere of 100% of oxygen;
(ii) atmosphere containing halogen elements within the oxygen atmosphere; and
(iii) atmosphere containing oxygen and having an oxidizing effect.
Normally, normal pressure is used as pressure of the atmosphere. However, it may be put into a state of reduced pressure or a pressurized state. It is also possible to introduce moisture.
The thin film semiconductor device may be formed directly on the thermal oxide film. However, it may be structured by depositing an insulating film further. It is also possible to arrange such that a film having high thermal conductivity such as a carbon film or an aluminum nitride film is provided as a heat radiating layer and the semiconductor device is formed thereon.
(6) According to a still other arrangement of the invention, there is provided a method for fabricating a substrate of a semiconductor device, comprising steps of depositing an amorphous silicon film at least on main face of the substrate; and transforming the amorphous silicon film into a thermal oxide film by heating in oxidizing atmosphere.
The invention disclosed in the present specification is useful not only in the fabrication steps of the semiconductor device but also in the method for fabricating the substrate used in the semiconductor device.
In the arrangements (5) and (6), a substrate having heat resistance such as a quartz substrate and a crystallized glass substrate which can sustain the heat in forming the thermal oxide film may be used. Further, as the substrate, a low grade mono-crystal silicon substrate (mono-crystal silicon wafer) or a poly-crystal silicon substrate (poly-crystal silicon wafer) may be used.
(7) Further, in order to solve the above-mentioned problems, according to another arrangement of the invention disclosed in the present specification, there is provided a method for fabricating a semiconductor device, comprising steps of forming an insulating film on the exposed surrounding surface of a glass or quartz substrate by means of reduced pressure thermal CVD; depositing an amorphous silicide film so as to cover the insulating film by means of reduced pressure thermal CVD; depositing an insulating film having the same quality with the insulating film so as to cover the silicide film by means of reduced pressure thermal CVD; and completing a thin film transistor by using the insulating film deposited so as to cover the silicide film in the previous step as a gate insulating film.
(8) According to another arrangement of the invention, there is provided a method for fabricating a semiconductor device, comprising steps of depositing an insulating film on the exposed upper, back and side faces of a glass or quartz substrate by means of reduced pressure thermal CVD; depositing an amorphous silicide film so as to cover the insulating film by means of reduced pressure thermal CVD; depositing an insulating film having the same quality with the insulating film so as to cover the silicide film by means of reduced pressure thermal CVD; and completing a thin film transistor by using the insulating film deposited so as to cover the silicide film in the previous step as a gate insulating film.
As the amorphous silicide film, an amorphous silicon film or an amorphous film represented as SiXGe1-X (0xe2x89xa6Xxe2x89xa61) may be used. As the blocking layer, a film selected from a silicon oxide film, a silicon nitride film and a silicon oxide nitride film may be used.
As a method for depositing the silicon oxide film, the reduced pressure thermal CVD using silane and oxygen or dichlorosilane and oxygen as original gases may be used.
As a method for depositing the silicon nitride film and the silicon oxide nitride film, the reduced pressure thermal CVD using silane and N2O or silane and NO2 as original gases may be used. The silicon nitride film may be deposited by using the reduced pressure thermal CVD using dichlorosilane and ammonia.
Specifically, plasma CVD using dichlorosilane and ammonia has an effect that it allows a film having less defects to be formed because the defects within the film to be deposited are terminated by chlorine.
(9) According to another arrangement of the invention, there is provided a semiconductor device comprising a thin film transistor formed on one face of a glass substrate, wherein an insulating film composing a gate insulating film of the thin film transistor is deposited so as to surround the glass substrate.
(10) According to another arrangement of the invention, there is provided a semiconductor device comprising a thin film transistor formed on one face of a glass substrate, wherein an insulating film composing a gate insulating film of the thin film transistor is deposited also on the back of the glass substrate.
(11) According to another arrangement of the invention, there is provided a semiconductor device utilizing a thin film transistor formed on one face of a glass substrate, wherein an insulating film composing a gate insulating film of the thin film transistor is deposited also on the back of the glass substrate.
In the above-mentioned three arrangements (9) through (11) of the invention, the insulating film is deposited on the bottom face of an active layer composing the thin film transistor and is also deposited on the back of the glass substrate.
(12) According to another arrangement of the invention, there is provided a semiconductor device utilizing a thin film transistor formed on one face of a glass substrate, wherein an insulating film composing a gate insulating film of the thin film transistor has the same component with an insulating film deposited under an active layer of the thin film transistor and the insulating film is deposited also on the back of the glass substrate.
(13) According to another arrangement of the invention, there is provided a semiconductor device utilizing a thin film transistor formed on one face of a glass substrate, wherein an insulating film composing a gate insulating film of the thin film transistor has the same component with an insulating film deposited under an active layer of the thin film transistor and the insulating film deposited under the active layer is deposited so as to surround the glass substrate.
In the above-mentioned arrangement (13), it is effective to contain halogen element within the insulating film. For instance, chlorine may be included in the film in depositing the insulating film by using dichlorosilane. Concentration of the halogen element at this time is desirable to be less than 5 atom %.
The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings in which like numerals refer to like or corresponding parts.